Implement more methods of the ArangoDB's REST API marked for milestone 2 (M.2).

The rincon_client crate does not implement all methods of ArangoDB's REST API. The current status is documented in docs/arangodb_rest_api_methods.md. Methods that are planned to be implemented are marked with a milestone.

The way we handle authorization in our resolvers is pretty nice and declarative. We should open source this module and perhaps even write a little blog post.

https://github.com/mikestaub/slack-lunch-club/blob/master/backend/packages/graphql/schema/utils.js